PCT/US2015/48710, filed Sep. 4, 2015, and U.S. Provisional Application No. 62/045,872, filed Sep. 4, 2014, are incorporated herein by reference in their entirety.
There are many devices and methods of imparting a force (or torque) to a moving object. In some cases it is beneficial to control that force very precisely and at the same time dynamically monitor the position of the object, again with some degree of precision. Various servomotors, servo-controlled fluid systems, motor/encoder combinations can accomplish that task suitably for many applications. However, if the speed of motion is slow, or if the controlled force needs to be selectable from a relatively low value to a much larger value, then conventional systems can fail to hold the desired output force to a precise or consistent value. Also, some systems may be moving sufficiently fast but are subject to encountering sudden or widely varying resistance, such when the object impacts some fixed object. In those applications conventional systems such as those mentioned above, can lack the fast response time required to prevent a spike in the level of force imparted.
Also the nature of motion control systems is that changes to motor position cannot be instantaneous, and although properly tuned, there can still be overshoot and undershoot in the resulting applied force or torque which in a purely rigid system would result in brief fluctuations in the force or torque delivered to the moving object.
In the context of bore finishing, such as, but not limited to, honing, a finishing tool assembly will be rotated within a bore or other cavity of a workpiece, about an axis and reciprocatingly moved or stroked along the axis, while a feed force is applied radially or laterally to an abrasive element or elements, e.g., honing stone or stones, for abrading material from the surface of the workpiece within the bore or cavity. This feed force is applied by a feed mechanism or system which typically includes one or more feed elements within the tool, e.g., wedge or wedges of the finishing tool assembly. The feed force is applied by an output rod of the feed mechanism axially against the wedge or wedges, which translate the axial force into radial outward force applied to the honing stone or stones. The applied force is typically precise, but can be intentionally varied as required for imparting certain precise characteristics to the surface of the workpiece bore or cavity. As an example, the workpiece bore or cavity may originally have a shape such as a barrel, hourglass or taper, and it may be sought to remove that original shape and impart a very precise cylindrical shape to the surface. The original bore may also skew, or have other malformations that are required to be removed by the honing or other bore finishing operation. Conversely, it may be desired to impart a precise taper, hourglass, or barrel shape. In both instances, it may be necessary to precisely vary the feed force applied to the honing stone or stones, and also varying the stroking action, to abrade the surface of the bore or cavity in a manner to achieve the sought after characteristics. The feed force can thus desirably be sought to be a precisely controlled constant force, or a precisely controlled variable force, e.g., controlled to virtually any required function or algorithm, for instance, based on time, position, or other variables that are monitored by the control system controlling operation of the feed mechanism.
Generally, in a bore finishing machine, feed force is generated using drive apparatus of the feed system, such as a servomotor, controlled by a motion control system capable of responding to externally measured or computed digital signals. Some mechanism is used to convert the motor rotation to linear motion. As examples, the mechanism can be a ball screw and ball nut, a rack and pinion, or any other device capable of converting rotary motion to linear motion. The ball nut, moving linearly, pushes against a spring which in turn bears against the output rod. An alternative is a linear actuator, cylinder, or the like, can be used.
Reference Cloutier et al., U.S. Pat. No. 7,575,502, entitled method of operating honing feed system having full control of feed force, rate, and position; and Cloutier et al., U.S. Pat. No. 8,277,280, entitled honing feed system and method employing rapid tool advancement and feed force signal conditioning, which discuss various feed control principles and apparatus, the disclosures of which are incorporated herein by reference in their entireties. While these system and methods provide satisfactory performance, they are complex and costly to implement.
A problem that can occur when operating less complex and costly feed systems when attempting to apply precise feed force, whether constant or varied in a controlled manner, is that as the tool encounters obstacles, irregularities, bore size variances, such as a narrower bore section, surface skew, or the like, rigidity of the feed mechanism can cause a feed force spike if the feed force is not sufficiently quickly adjusted or attenuated. This is problematic as feed and motion control systems always require some amount of time to react to sudden changes and in that amount of time the force will continue to increase which can result in variations, longer process times, and other shortcomings or problems.
Also the nature of motion control systems such as feed systems is that changes to motor position cannot be instantaneous, and although properly tuned, there can still be overshoot and undershoot which in a purely rigid system would result in brief fluctuations in the force delivered by the output rod.
Thus what is sought is a manner of force controlled motion, to substantially reduce or eliminate the above referenced problems, with minimal complexity and cost compared to more sophisticated known feed control systems.